The manufacture of water-swellable polymers is known. A typical method of producing water-swellable polymer particles by inverse suspension polymerization of an aqueous solution of acrylic acid and alkali or ammonium acrylate in a hydrocarbon, and then cross-linking the polymer is described in U.S. Pat. No. 4,698,404.
European patent application No. A1 0 205 674 describes the solvent polymerization of monomers substantially containing acid groups in the presence of an agent having a cross-linking action. After the acid groups of the polymer have been neutralized by at least 27% and the polymer gel has been dried, a water-swellable polymer is obtained which has a high gel strength and small amounts of extractable components.
European patent application No. A2 0 312 952 describes the polymerization of carboxyl-groups-containing monomers neutralized by at least 20% in the presence of dispersed cross-linking agents.
The continuous copolymerization of a monomer solution containing an acrylic acid, in which the acrylic acid is present to the extent of at least 20% in a neutralized form, is described in European Application A2 0 280 541.
In order to improve the application properties of water swelling polymers selected from the group consisting of a crosslinked hydrolysate of an acrylamidecopolymer, and a hydrolysate of a crosslinked acrylnitrilecopolymer, a crosslinked, partially neutralized acrylic acid polymers, or to adapt them to a specific application, they are frequently post-treated.
European patents 0 009 977 and 0 001 706 described the surface coating of water swelling polymers in the particle form, with hydrophobic compounds or polythers, with the purpose of improving their dispersion in blood.
U.S. Pat. No. 4,587,308 describes the post-crosslinking of water swelling polymers, dispersed in the swelled form, with diglycideethers, polyvalent metal salts or halogen-epoxy compounds.
DE-OS 34 29 379 describes a water absorbing resin, obtainable by the impregnation of a water swelling resin with a hydrophilic, reactive, olefinically unsaturated monomer or a polymer of it, and a crosslinking agent.
U.S. Pat. No. 4,666,975 describes the post-cross-linkage of water-swellable polymers in suspension by reaction with diglycide ethers.
GB 21 19 384 A claims the secondary cross-linkage in the particle surface of water-swellable polymers, the carboxyl groups of which are neutralized to 50 to 99 mol-%, with multifunctional compounds, such as polyols or polyamines.
The secondary cross-linkage in the polymer surface with solutions of polyols is described in DE-OS 35 23 617. The aftertreatment is carried out with polymers whose carboxyl groups are neutralized by 50 to 99 mol-% in the presence of water and a hydrophilic organic solvent.
All these processes strive for improving the liquid transport in a layer of polymer particles or providing a polymer which, after swelling, does not release the stored liquid on exertion of pressure, e.g., by an infant's body on the swollen polymer within a diaper. However, it is the object not only to improve the liquid transport in a layer of water-swellable polymer particles or the capability of the polymer to retain liquid under subsequent pressure when the polymer could swell freely, i.e., without load, but also to provide a water-swellable polymer which is capable of absorbing liquid even against a load. This special absorption property is referred to as absorption under pressure/load in the European Patent 0 339 461.